What is this supposed to be?

[triangular_cube]

Does the scenario book identify it as an APC for game purposes as the counter does?

The scenario book does call it an APC. In parallel to Felipe's point, the M20 is an M8 with the turret removed and the crew compartment roof mostly removed. The vehicle normally has a crew of 4, certainly not room for a squad of troops. As a side note, whom ever entered the library entry, though it has mechanized movement.
 

Regardless of what it ACTUALLY is, I would follow those directions as to how to play it. 

Now the question of simulation vs. game play comes in. If the game M20 is a misnomer for some actual APC, the question remains, what is it really? I don't know of any wheeled APC in the U.S. inventory at this time that it could be. The M20, elsewhere in the PG-verse is a scout car, in reality it was a scout car. Does this mean we are playing with a made-up vehicle for game purposes?

Six wheels = motorized, not mechanized, que no?

I would think so.